Understood to be more beautiful than plain "pretty," but not gorgeous enough to gain the "beautiful" title. Cutsy way of saying "kinda hott."
Also can be used in describing objects, meaning that an object is very colorful and random.
"Oh my god, Kaitlin, he would never go out with me. He likes her. I mean, shes just so prettiful, and I'm just so...not!"
"Ooh, I love your shirt, it's very prettiful!